Impossible crystals

21-03-2013

For 70 years following the discovery of X-ray crystal diffraction, the textbook defi nition of a crystal remained unchallenged: a crystal is an ordered, periodic structure. But in 1982, Dan Shechtman at the Technion Israel Institute of Technology rocked the world of crystallography with the discovery of the first ordered but nonperiodic crystal: a quasicrystal, which has higher crystal symmetries than are possible in periodic systems. Hundreds of quasiperiodic crystals have since been discovered, and last year, having initially faced strong opposition for his finding, Shechtman won the 2011 Nobel Prize for Chemistry.

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Quasicrystals have been used to reinforce steel and polymer composites and even to make non-stick coatings for frying pans. But it is their impossible structure that attracts physicists. Quasicrystals were discovered using transmission electron microscopy, and it wasn’t until a few years later that crystals were grown large enough to be studied using X-rays. Since the beginning, explains ESRF user Marc de Boissieu of the CNRS in Grenoble, the ESRF and the ILL have played an active role in understanding quasicrystals. “This has been a long story that led to key results in the understanding of the structure and the dynamics of quasicrystals.”

Diffuse X-ray scattering measurements have been vital in understanding fluctuations specific to quasicrystals called phasons. Another prominent result, says de Boissieu, was a study performed at the French CRG beamline BM02 in 2007, which revealed the structure of the binary cadmium-ytterbium quasicrystal (left, bottom; Nature Materials 6 58).

“The high-quality data acquired allowed us to solve, for the first time, the structure of a quasicrystal at a level comparable to what is achieved in standard crystallography,” he explains. “This paved the way to the understanding of physical properties of their formation.” The Swiss–Norwegian CRG beamline, meanwhile, has provided a much better understanding of decagonal phases and, more recently, of a soft matter quasicrystal with 12-fold and 18-fold symmetry, while experiments at ID19 have imaged the in situ growth of quasicrystals.

On 18–19 October, Shechtman and 20 others met in Grenoble to discuss the state of the art in quasicrystal research. “Whereas the structure of quasicrystals and some of their physical properties are well understood, the mechanisms bringing in the long range quasiperiodic order remains one of the big questions,” says de Boissieu.

 

 

This article originally appeared in ESRFnews, December 2012. 

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Top image: Diffraction pattern (top) and structure of icosahedral quasicrystals.